This invention relates to a foamable rubber composition and a foamed rubber. More particularly, it relates to a foamable rubber composition having enhanced processability and giving a foamed and shaped rubber article having enhanced hardness, and good shrink-resistance tear-resistance and abrasion-resistance; and the foamed and shaped rubber article.
A foamed rubber article has a reduced specific gravity and thus is light-weight, and exhibits good impact absorption and heat-insulation, and therefore, is used in many fields. As rubber materials for foamed rubber articles, there can be mentioned natural rubber, and synthetic rubbers such as a polyisoprene rubber, an acrylonitrile-butadiene copolymer rubber, a styrene-butadiene copolymer rubber, an ethylene-propylene copolymer rubber and an ethylene-vinyl acetate copolymer rubber.
Recently, there was proposed a foamed rubber article made from a foamable rubber composition comprising a hydrogenated acrylonitrile-butadiene copolymer rubber having incorporated therein a zinc salt of an ethylenically unsaturated carboxylic acid, a metal salt of a higher fatty acid, a foaming agent and an organic peroxide (for example, Japanese Unexamined Patent Publication No. H6-107740). This foamable rubber composition was proved to give a foamed and shaped rubber product exhibiting a large shrinkage after crosslinked or after heat-treated in the course of making the foamed and shaped rubber article, namely, the rubber composition is difficult to shape into a foamed and shaped rubber article.
An object of the present invention is to provide a foamable rubber composition giving a foamed and shaped rubber article exhibiting no shrinkage after crosslinked or after heat-treated in the course of making the foamed and shaped rubber article, the conditions for making the foamed and shaped rubber article from which composition are easy to control at the step of making the foamed and shaped rubber article, and which composition exhibits good foamability.
Another object of the present invention is to provide a foamed and shaped rubber article exhibiting improved tear-resistance, abrasion-resistance and other properties, and having large elongation and high hardness.
The present inventors made extensive researches to achieve the above-mentioned objects and found that a foamable rubber composition comprising a nitrile group-containing highly saturated copolymer rubber and polyethylene exhibited enhanced processability and foamability, and gave a foamed and shaped article exhibiting good shrink- resistance, tear-resistance and abrasion resistance and having large elongation and high hardness. Based on this finding, the present invention has been completed.
In accordance with the present invention, there is provided a foamable rubber composition comprising a nitrile group-containing highly saturated copolymer rubber, polyethylene, a metal salt of an ethylenically unsaturated carboxylic acid, an organic peroxide and a foaming agent.
In accordance with the present invention, there is further provided a foamed and shaped rubber article made by foaming and shaping the above-mentioned foamable rubber composition,
The nitrile group-containing highly saturated copolymer rubber is a rubber having an iodine value not larger than 120 obtained by copolymerization of an xcex1, xcex2-ethylenically unsaturated nitrile monomer with other monomer or monomers. Iodine value is an indication demonstrating the degree of unsaturation of carbon-carbon bond, and expressed in terms of amount in gram of iodine capable of being added to 100 g of a rubber.
The monomer to be copolymerized with an xcex1, xcex2-ethylenically unsaturated nitrile monomer includes, for example, conjugated diene monomers, non-conjugated diene monomers and xcex1-olefin monomers. In the case where an xcex1, xcex2-ethylenically unsaturated nitrile monomer is copolymerized with a conjugated dione monomer, a copolymer rubber having too large iodine value tends to be produced. When a copolymer rubber having too large iodine value is produced, the copolymer rubber may be subjected to hydrogenation addition whereby the carbon-carbon unsaturated bonds in the copolymer rubber are hydrogenated and the iodine value of copolymer rubber is lowered.
The xcex1, xcex2-ethylenically unsaturated nitrile monomer includes, for example, acrylonitrile, methaorylonitrile and xcex1-chloroacrylonitrile. Of these, acrylonitrile is, preferable. These monomers may be used either alone or in combination.
The lower limit of the content of xcex1, xcex2-ethylenically unsaturated nitrile monomer units in the nitrile group-containing highly saturated copolymer rubber is preferably 10% by weight, and the upper limit thereof is preferably 60% by weight, more preferably 55% by weight and especially preferably 50% by weight.
As specific examples of the conjugated diene monomer, there can be mentioned 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene and 1,3-pentadiene. Of these, 1,3-butadiene is preferable. In the case where the nitrile group-containing highly saturated copolymer rubber is a copolymer of an xcex1, xcex2-ethylenically unsaturated nitrile monomer, a conjugated diene monomer and an optional copolymerizable monomer or monomers, or its hydrogenation product, the lower limit of the content of conjugated dione monomer units in the copolymer rubber is preferably 30% by weight, more preferably 40% by weight and especially preferably 30% by weight, and the upper limit thereof is preferably 90% by weight.
The non-conjugated dione monomer preferably includes those which have 5 to 12 carbon atoms, such as, for example, 1,4-pentadiene and 1,4-hexadiene.
The xcex1-olefin monomer preferably includes those which have 2 to 12 carbon atoms, such as, for example, ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene.
The optional monomer copolymerizable with an xcex1, xcex2-ethylenically unsaturated nitrile monomer includes, for example, unsaturated carboxylic acid esters, aromatic vinyl monomers, fluorine-containing vinyl monomers, unsaturated monocarboxylic acids, unsaturated dicarboxylic acids and anhydrides thereof, and copolymerizable antioxidants.
As specific examples of the unsaturated carboxylic acid esters, there can be mentioned alkyl acrylates and alkyl methacrylates, which have 1 to 18 carbon atoms in the alkyl group, such as methyl acrylate, ethyl acrylate, n-dodecyl acrylate, methyl methacrylate and ethyl methacrylate; alkoxyalkyl acrylates and alkoxyalkyl methacrylates, which have 2 to 12 carbon atoms in the alkoxyalkyl group, such as methoxymethyl acrylate and methoxyethyl methacrylate; cyanoalkyl acrylates and cyanoalkyl methacrylates, which have 2 to 12 carbon atoms in the cyanoalkyl group, such as xcex1-cyanoethyl acrylate, xcex2-cyanoethyl acrylate and cyanobutyl methacrylate; hydroxyalkyl acrylates such as 2-hydroxyethyl acrylate and hydroxypropyl acrylate; monoalkyl or dialkyl esters of unsaturated dicarboxylic acids such as monoethyl maleate, dimethyl maleate, dimethyl fumarate, dimethyl itaconate, n-butyl itaconate and diethyl itaconate; amino group-containing unsaturated carboxylic acid esters such as dimethylaminomethyl acrylate and diethylaminoethyl acrylate; fluoroalkyl group-containing acrylates and fluoroalkyl group-containing methacrylates, such as trifluoroethyl acrylate and tetrafluoropropyl methacrylate; and fluoro-substituted benzyl acrylates and fluoro-substituted benzyl methacrylates, such as fluorobenzyl acrylate and fluorobenzyl methacrylate.
As specific examples of the aromatic vinyl monomer, there can be mentioned styrene, xcex1-methylstyrene and vinylpyridine.
As specific examples of the fluorine-containing vinyl monomer, there can be mentioned fluoroethyl vinyl ether, fluoropropyl vinyl ether, o-trifluoromethylstyrene, vinyl pentafluorobenzoate, difluoroethylene and tetrafluoroethylene. As specific examples of the unsaturated monocarboxylic acid, there can be mentioned acrylic acid and methacrylic acid. As specific examples of the unsaturated dicarboxylic acid and its anhydride, there can be mentioned itaconic acid, fumaric acid and maleic acid, and maleic anhydride, respectively. As specific examples of the copolymerizable antioxidant, there can be mentioned N-(4-anilionophenyl)acrylamide, N-(4-anilionophenyl)methacrylamide, N-(4-anilionophenyl)-cinnamamide, N-(4-anilionophenyl)crotonacrylamide, N-phenyl-4-(3-vinylbenzyloxy)aniline and N-phenyl-4-(4-vinylbenzyloxy)aniline. These copolymerizable monomers may be used as a combination of at least two thereof.
The lower limit of the Mooney viscosity (ML1+4, 100xc2x0 C.) of the nitrile group-containing highly saturated copolymer rubber used in the present invention is preferably 15, more preferably 30 and especially preferably 45, and the upper limit thereof is preferably 200 and more preferably 100. When the Mooney viscosity is too small, a foamed and shaped rubber article of the present invention has poor mechanical strength. In contrast, when the Mooney viscosity is too large, a foamable rubber composition of the present invention has poor processability.
The upper limit of the iodine value of the nitrile group-containing highly saturated copolymer rubber is preferably 120, more preferably 80 and especially preferably 50. When the iodine value is too large, balance between elongation and strengths of a foamed and shaped rubber article of the invention becomes bad.
Polyethylene
Polyethylene used in the present invention is a homopolymer of ethylene or a copolymer of ethylene with an xcex1-olefin monomer. The ethylene/xcex1-olefin copolymer is preferably a copolymer containing not more than 5% by mole of xcex1-olefin monomer units. As specific examples of the polyethylene, there can be mentioned low-density polyethylene having long chain branches produced by a radical polymerization carried out under a high pressure, for example, about 1,000 atmospheric pressure; straight-chain high-density polyethylene produced by a coordination anion polymerization carried out in the presence of a transition metal catalyst under a low pressure for example, about 50 atmospheric pressure; and straight-chain low-density polyethylene produced by copolymerization with an xcex1-olefin monomer carried out in the presence of a transition metal catalyst under a low pressure, for example, about 50 atmospheric pressure.
The density of polyethylene is not particularly limited, but, the lower limit thereof is preferably 0.91 Mg/m3 (g/cm3), more preferably 0.94 Mg/m3, and the upper limit thereof is preferably 1.00 Mg/m3. When specific gravity of polyethylene is too small, a foamed and shaped rubber article of the present invention has poor tear resistance and abrasion-resistance. In contrast, when specific gravity of polyethylene is too large, the polyethylene becomes difficult to produce.
The melt flow rate of polyethylene as measured at 190xc2x0 C. and 21.18 N under testing condition 4 of JIS K7210 is not particularly limited, but, the lower limit of the melt flow rate is preferably 0.001 g/10 min., more preferably 0.1 g/10 min., and the upper limit thereof is preferably 30 g/10 min. When the melt flow rate is too small, a foamable rubber composition of the present invention tends to exhibit poor processability and a foamed and shaped rubber article is difficult to produce. In contrast, when the melt flow rate is too large, the polyethylene is difficult to produce and the foamed and shaped rubber article has poor physical properties.
The lower limit of the amount of polyethylene is, based on 100 parts by weight of the nitrile group-containing highly saturated copolymer rubber, preferably 5 parts by weight, and the upper limit thereof is preferably 150 parts by weight, more preferably 100 parts by weight and especially preferably 50 parts by weight. When the amount of polyethylene is too large, kneading of the rubber composition becomes difficult. In contrast, the amount thereof is too small, a foamed and shaped rubber article has poor mechanical strengths.
An ethylenically unsaturated carboxylic acid for the preparation of a metal salt thereof used in the present invention has at least one free carboxyl group for forming the metal salt and includes, for example, unsaturated monocarboxylic acids, unsaturated dicarboxylic acids and monoesters of unsaturated dicarboxylic acids. As specific examples of the ethylenically unsaturated carboxylic acid, there can be mentioned unsaturated monocarboxylic acids such as acrylic acid and methacrylic acid, unsaturated dicarboxylic acids such as maleic acid, fumaric acid and itaconic acid; and monoesters of unsaturated dicarboxylic acids such as monomethyl maleate, monoethyl maleate, monomethyl itaconate and monoethyl itaconate.
Of these, ethylenically unsaturated carboxylic acids having no eater group are preferable in view of the mechanical strengths of a foamed and shaped rubber article. An unsaturated monocarboxylic acid and an unsaturated dicarboxylic acid are more preferable. An unsaturated monocarboxylic acid is especially preferable.
The metal for the preparation of the metal salt of an unsaturated carboxylic acid is not particularly limited provided that it is capable of forming a salt with the above-mentioned unsaturated carboxylic acid, but, in view of the mechanical strengths of a foamed and shaped rubber article, it is preferably selected from zinc, magnesium a calcium, barium, titanium, chromium, iron, cobalt, nickel, aluminum, tin and lead, more preferably from zinc, magnesium, calcium and aluminum, and especially preferably from zinc and magnesium.
The lower limit of the content of metal for the preparation of the metal salt of an ethylenically unsaturated carboxylic acid is, per mole of carboxyl group in the ethylenically unsaturated carboxylic acid, is preferably 0.2 mole, more preferably 0.3 mole and especially preferably 0.4 mole, and the upper limit thereof is preferably 3 moles, more preferably 2 moles and especially preferably 1 mole. When the relative amount of metal to an ethylenically unsaturated carboxylic acid is too small, a foamable rubber composition and a foamed and shaped rubber article give off an offensive smell of residual monomer. In contrast, when the relative amount of metal to an ethylenically unsaturated carboxylic acid is too large, a foamed and shaped rubber article has poor mechanical strengths.
The method of preparing the foamable rubber composition by mixing and kneading together the metal salt of an ethylenically unsaturated carboxylic acid with rubber and other ingredients includes, for example, is a method of mixing the metal salt of an ethylenically unsaturated carboxylic acid with rubber and other ingredient, (ii) a method of mixing an ethylenically unsaturated carboxylic acid with an oxide, hydroxide or carbonate of metal and allowing these ingredients to, react with each other during kneading to produce the metal salt of an ethylenically unsaturated, carboxylic acid, and (iii) a method of preparing a master batch of the metal salt of an ethylenically unsaturated carboxylic acid and mixing the master batch with rubber and other ingredients at a proportion such that a rubber composition of the present invention having the desired metal salt content is prepared.
The lower limit of the amount of the metal salt of an ethylenically unsaturated carboxylic acid its, based on 100 parts by weight of the nitrile group-containing highly saturated copolymer rubber, preferably 3 parts by weight, more preferably 5 parts by weight and especially preferably 10 parts by weight, and the upper limit thereof is preferably 100 parts by weight, more preferably 70 parts by weight and especially preferably 50 parts by weight. When the relative amount of the metal salt of an ethylenically unsaturated carboxylic acid to the nitrile group-containing highly saturated copolymer rubber is too small, a foamed and shaped rubber article has poor mechanical strengths. In contrast, when the relative amount of the metal salt of an ethylenically unsaturated carboxylic acid to the nitrile group-containing highly saturated copolymer rubber is too large, a foamed and shaped rubber article has poor elongation.
Organic Peroxide
The organic peroxide used in the prevent invention is not particularly limited, and those which are used in a rubber industry as organic peroxide crosslinking agents for crosslinking rubber and polyethylene may be used. The organic peroxide includes, for example, dialkyl peroxides, diacyl peroxides and peroxy eaters. As specific examples of these organic peroxides, there can be mentioned dialkyl peroxides such as dicumyl peroxide, di-t-butyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)-3-hexyne, 2,5-dimethyl-2,5-di(t-butylperoxy)-3-hexane and 1,3-bis(t-butylperoxyisopropyl)benzene; dialkyl peroxides such as benzoyl peroxide and isobutyryl peroxide; and peroxy esters such as 2,5-dimethyl-2,5-bis(benzoylperoxy)hexane and t-butylperoxyisopropyl carbonate. These organic peroxides may be used either alone or in the form of being dispersed in, for example, clay, calcium carbonate or silica. When an organic peroxide is used in the latter form, a foamable rubber composition exhibits enhanced processability. The, organic peroxides may be used as a combination of at least two thereof.
The lower limit of the amount of organic peroxide is, based on 100 parts by weight of the nitrile group-containing highly saturated copolymer rubber, preferably 0.5 part by weight, and the upper limit thereof is preferably 10 parts by weight, more preferably 8 parts by weight and especially preferably 6 parts by weight. When the relative amount of organic peroxide to the nitrile group-containing highly saturated copolymer rubber is too small, crosslinking reaction does not occur to the desired extent at the step of producing a foamed and shaped rubber article. In contrast, when the relative amount of organic peroxide to the nitrile group-containing highly saturated copolymer rubber is too large, a foamed and shaped rubber article is hard and has poor rubber elasticity.
Foaming Agent
The foaming agent used in the present invention is not particularly limited, and any foaming agent can be adopted provided that it is used in a rubber industry. The foaming agent includes, for example, azo compound foaming agents, sulfonylhydrazide compound foaming agents and amine compound foaming agents, of these, azo compound foaming agents are preferable because they do not give no baneful influence upon the crosslinking caused by an organic peroxide. As specific examples of the foaming agents, there can be mentioned azo compound foaming agents such as azodicarbonamide and azobisisobutyronitrile; sulfonylhydrazide compound foaming agents such as benzenesulfonylhydrazide, p-toluenesulfonylhydrazide and 4,4-oxybis-(benzenesulfonylhydrazide) and amine compound foaming agents such as N,N-dinitrosopentamethylenetetramine.
The foaming agent may be used either alone or as a combination of at least two thereof. An ingredient for lowering the decomposition temperature of the foaming agent may be incorporated which includes, for example, zinc oxide, urea and ethanolamine.
The lower limit of the amount of a foaming agent is, based on 100 parts by weight of the nitrile group-containing highly saturated copolymer rubber, preferably 1 part by weight, and the upper limit thereof is preferably 30 parts by weight, more preferably 20 parts by weight and especially preferably 15 parts by weight. When the amount of a foaming agent is too small, foaming does not occur to the desired extent and a resulting foamed and shaped rubber article is not characterized as foamed rubber. In contrast, when the amount of a foaming agent is too large, the extent of foaming becomes excessively large and the size of cells becomes non-uniform and too large, and the resulting foamed and shaped rubber article has extremely reduced mechanical strengths and abrasion resistance.
Other Ingredients
According to the need, the foamable rubber composition of the present invention may have further incorporated therein ingredients other than the above-mentioned ingredients, which are used in a rubber industry and include, for example, a reinforcer such as carbon black, silica or talc, a filler such as calcium carbonate or clay, a processing aid, a plasticizer, an antioxidant, an antiozonant, a co-crosslinking agent and a colorant. The amount of these optional ingredients may be appropriately chosen so that the intended function can be achieved while the object of the present invention is not impeded.
Especially, a metal salt of a higher fatty acid is preferably incorporated in the foamable rubber composition of the present invention. By the incorporation of a higher fatty acid metal salt, the foamable rubber composition exhibits a reduced viscosity and enhanced processability.
A higher fatty acid having a carboxyl functional group, which forms the higher fatty acid metal salt, has carbon atoms except for the carboxyl functional group, the lower limit of which is preferably 6, more preferably 8 and especially preferably 10 and the upper limit of which is preferably 30, more preferably 25 and especially preferably 20. When the number of carbon atoms except for the carboxyl functional group of higher fatty acid is too small, the higher fatty acid is liable to volatilize and give off an offensive small when exposed to a high temperature during kneading or foaming and shaping, and, when the content of the higher fatty acid metal salt is excessively lowered, the releasability of the foamed and shaped rubber article from a mold tends to become poor. In contrast, when the number of carbon atoms except for the carboxyl functional group of higher fatty acid is too large, the higher fatty acid metal salt has a high melting point and workability at the step of kneading is liable to become poor.
The higher fatty acid includes saturated higher fatty acids and unsaturated higher fatty acids. As specific examples of the saturated higher fatty acids, there can be mentioned caproic acid, caprylic acid, capric acid, lauric acid, tridecylic acid, myristic acid, pentadecylic acid, stearic acid, nonadecanoic acid, arachic acid, behenic acid, lignoceric acid, cerotic acid, montanic acid and melissic acid. As specific examples of the unsaturated highest fatty acids, there can be mentioned oleic acid, elaidic acid, linolic acid, linolenic acid and arachidonic acid.
The metal of the higher fatty acid metal salt is not particularly limited, but is preferably selected from metals of group 1, group 2, group 12 and group 14 of the periodic table. As specific examples of the metal, there can be mentioned metals of group 1 such as lithium, potassium and sodium; metals of group 2 such as magnesium, calcium and barium; metals of group 12 such as zinc and cadmium, and metals of group 14 such as tin and lead. More preferably the metal is selected from lithium, potassium, sodium, magnesium, calcium, zinc and tin. Magnesium, calcium, zinc and tin are especially preferable.
The lower limit of the amount of the higher fatty acid metal salt is, based on 100 parts by weight of the nitrile group-containing highly saturated copolymer rubber, preferably 0.1 part by weight, more preferably 0.5 part by weight and especially preferably 0.7 part by weight, and the upper limit thereof is preferably 20 parts by weight, more preferably 15 parts by weight and especially preferably 10 parts by weight. When the amount of the higher fatty acid metal salt is too small, a foamed and shaped rubber article has poor releasability from a mold. In contrast, when the amount of the higher fatty acid metal salt is too large, a foamed and shaped rubber article exhibits an undesirably reduced crosslink density.
Foamable Rubber Composition
The foamable rubber composition of the present invention comprises a nitrile group-containing highly saturated copolymer rubber, polyethylene, a metal salt of an ethylenically unsaturated carboxylic acid, an organic peroxide and a foaming agent. According to the need, the foamable composition may further comprise various ingredients as optional ingredients other than these ingredients.
Method of Preparing Foamable Rubber Composition
The foamable rubber composition of the present invention is prepared by kneading together a nitrile group-containing highly saturated copolymer rubber, polyethylene, a metal salt of an ethylenically unsaturated carboxylic acid, an organic peroxide, a foaming agent and optional ingredients by using an ordinary kneader such as a roll mill or a closed type kneader.
The order in which the respective ingredients are mixed together at the step of kneading is not particularly limited unless crosslinking or foaming occurs during kneading. For example, the respective ingredients are mixed together in the following order. First, a nitrile group-containing highly saturated copolymer rubber is kneaded with a metal salt of an ethylenically unsaturated carboxylic acid by a roll mill at a temperature (kneaded mixture temperature) of 140xc2x0 C. to prepare a master batch. Then the master batch is kneaded with polyethylene at a temperature (kneaded mixture temperature) of 140xc2x0 C. during which optional ingredients incapable of being decomposed or 140xc2x0 C. are incorporated in the kneaded mixture, until solid polyethylene disappears. The resulting mixture is cooled to room temperature, and then an organic peroxide, a foaming agent and optional ingredients capable of being decomposed at 140xc2x0 C. are incorporated therein and kneaded together. This final kneading is carried out at a temperature such that the organic peroxide is not decomposed and the foaming agent does not foam and kneading can be easily carried out. Preferably the final kneading temperature (kneaded mixture temperature) is not higher than 100xc2x0 C.
Foamed and Shaped Rubber Article
The foamed and shaped rubber article of the present invention is made by foaming and shaping the above-mentioned foamable rubber composition. The lower limit of the extent of foaming is preferably 100%, more preferably 150% and especially preferably 200%, and the upper limit thereof is preferably 2,000%, more preferably 1,500% and especially preferably 1,000%. When the extent of foaming is too small, the benefits due to foaming are minimized, and the foamed and shaped rubber article is not characterized as a foamed rubber, i.e., it does not have a low specific gravity, good impact resistance and high heat insulation. In contrast, when the extent of foaming is too large, the foamed and shaped rubber article has non-uniform and undesirably large cells, and thus, exhibits very poor mechanical strengths and extremely reduced abrasion resistance.
By the term xe2x80x9cextent of foamingxe2x80x9d used herein, we mean a ratio of volume expansion occurring due to foaming, which is expressed by a ratio in percent of the specific gravity of an non-foamed foamable rubber composition to the specific gravity of a foamed and shaped rubber article. When the extent of foaming is large, the ratio of volume expansion occurring due to foaming at the step of foaming and shaping is large.
The lower limit of the hardness as expressed by type E durometer hardness is preferably 10, more preferably 20 and especially preferably 30, and the upper limit thereof is preferably 95, more preferably 90 and especially preferably 85.
The foamed and shaped rubber article of the present invention can be made by foaming and shaping the foamable rubber composition. Foaming and shaping are concurrently carried out for making the foamed and shaped rubber article. Shaping is carried out by an ordinary method employed in a rubber industry, which includes, for example, compression molding, transfer molding, injection molding and extruding. When the shaping is conducted under heated conditions, foaming and crosslinking simultaneously occur.
The foamed and shaped rubber article can be made at one step by foaming and shaping the foamable rubber composition. The temperature at which the foaming and shaping are carried out is preferably in the range of 140 to 180xc2x0 C. The time for which the foaming and shaping are carried out is appropriately chosen depending upon the particular shape, especially thickness, of the foamed dad shaped rubber article, and is preferably in the range of 2 to 30 minutes. By this operation, foaming of the foamable rubber composition is fundamentally completed.
After completion of the foaming and shaping, second crosslinking (secondary crosslinking) can be conducted to remove strain of a foamed and shaped rubber article or improve the properties of the foamed and shaped rubber article. The conditions employed for the second crosslinking are not particularly limited, but usually the second crosslinking is carried out at a crosslinking temperature of 50 to 180xc2x0 C. for a crosslinking time of 1 to 5 hours, in the second crosslinking, foaming does not substantially occur.